The effect of anisotropy of strength properties on borehole stability during drilling

Introduction. The UCS (unconfi ned compressive strength) parameter is the unconfined compressive strength of rock, which is a signifi cant physical parameter used to describe strength state of rocks. During drilling, load on wellbore is redistributed depending on the inclination angle. The research introduced the concept of anisotropy coefficient of strength, which is a quantitative characteristic that refl ects the degree of reduction in rock strength depending on the orientation of the samples in diff erent directions relative to the applied load.

Aim. The aim of the work is to consider the anisotropy of strength properties and identify its infl uence on the stability of the wellbore during drilling.

Materials and methods. This article presents a review of the most well-known and practically relevant methods for estimating UCS values, which describe measurements of uniaxial rock compression based on elastic, acoustic, and other physical and mechanical properties of rocks. The influence of ultimate strength calculated by different methods on failure gradients is compared. Based on the interpretation of core data, dependences of ultimate strength on core samples cut at different angles (0°, 30°, 45°, 60°, and 90°) are constructed. 

Results. Anisotropy of strength properties was researched, and its impact on borehole stability was identified. The orientation of the sawn samples was adjusted to match the zenith angles during drilling. A general trend in tensile strength variation depending on the zenith angle of drilling was demonstrated. A collapse gradient assessment using the Mohr-Coulomb method, using the tensile strength curve for standard conditions and accounting for rock strength anisotropy, was considered.

Conclusion. The results of the research make it possible to clarify the risks associated with drilling wells in open-hole instability intervals and to assess the stability of directional and horizontal wells. 

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